Horsepower
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
I think this is best understood by getting a grasp of the basic formulas and principles.
Firstly, HP is simply a unit of measure of Power, as is kW. People do often use "Brake HP" interchangeably with "HP" - where brake HP is intended to tell you what the actual power is available at the driven wheel rather than at the flywheel, as you lose a lot of power through the transmission.
Power = Torque x angular velocity (rpm)
Plain and simple, if you want more torque at the same RPM you need more power - irrespective of what kind of engine you are using.
Now the complication comes in as engines do not deliver power in a linear relationship to RPM. At a more fundamental level, the kind of fuel used (actually the Thermodynamic cycle but the 2 are related) is the key factor goeverning the shape of the power curve. We all know that Diesel engines deliver optimum power at a much lower RPM level, which is why they are ideal for trucks and heavy vehicles.
Depending on engine design and gearing the manufactures can, to within reason, shift the power curve to give you optimum performance in the band that the vehicle is expected to perform -eg cruiser, racer, MX bike, crotch rocket.
When riding a cruiser you are more interested in low rpm without having to shift gears all the time, i.e as stated above you are looking for good torque at the lower rpm range. Hey, maybe a Diesel powered cruiser is the way to go?
Firstly, HP is simply a unit of measure of Power, as is kW. People do often use "Brake HP" interchangeably with "HP" - where brake HP is intended to tell you what the actual power is available at the driven wheel rather than at the flywheel, as you lose a lot of power through the transmission.
Power = Torque x angular velocity (rpm)
Plain and simple, if you want more torque at the same RPM you need more power - irrespective of what kind of engine you are using.
Now the complication comes in as engines do not deliver power in a linear relationship to RPM. At a more fundamental level, the kind of fuel used (actually the Thermodynamic cycle but the 2 are related) is the key factor goeverning the shape of the power curve. We all know that Diesel engines deliver optimum power at a much lower RPM level, which is why they are ideal for trucks and heavy vehicles.
Depending on engine design and gearing the manufactures can, to within reason, shift the power curve to give you optimum performance in the band that the vehicle is expected to perform -eg cruiser, racer, MX bike, crotch rocket.
When riding a cruiser you are more interested in low rpm without having to shift gears all the time, i.e as stated above you are looking for good torque at the lower rpm range. Hey, maybe a Diesel powered cruiser is the way to go?
Road Master
Joined: Dec 2005
Posts: 1,078
Likes: 5
From: Hickory, NC
Fair question and I'll give an example.
BMW S1000RR runs a 9.68 @ 150mph in one test. Horsepower is 193 at 13,000 rpm, Torque is about 83ft lbs @ 9,750 rpm. **** poor torque, huh?
If it was shifted at 9750 rpm, it wouldn't come anywhere near that ET and trap speed.
If it was given different gearing but only allowed to rev to 9750rpm, it wouldn't come close to it's top speed.
So although torque may be first measurable, the horsepower is also measurable on a dyno as well.
BMW S1000RR runs a 9.68 @ 150mph in one test. Horsepower is 193 at 13,000 rpm, Torque is about 83ft lbs @ 9,750 rpm. **** poor torque, huh?
If it was shifted at 9750 rpm, it wouldn't come anywhere near that ET and trap speed.
If it was given different gearing but only allowed to rev to 9750rpm, it wouldn't come close to it's top speed.
So although torque may be first measurable, the horsepower is also measurable on a dyno as well.
While horsepower is displayed on a dyno chart, it NOT directly measurable on a dyno. The horsepower curve on all dynos is generated based on a mathematical calculation from the torque. It's a made up number.
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
You are right that you can't measure power directly with a single instrument, but the calculation is well documented and understood, and both variables (torque and rpm) are easily measured so not quite a "made up" number. Power is relevant - in fact it is the most relevant figure.
Elite HDF Member
Joined: Apr 2007
Posts: 3,766
Likes: 16
From: Roseville, CA
With regards to the numbers above, they don't really mean much to those of us who ride bikes that weigh 750+ lbs. and can't turn more than 6000 RPM. You don't need as much torque to acclerate a 450 lb. bike to speed quickly. Hence the reason the S1000RR is able to run in the 9s with torque numbers that are seemingly (to us Harley riders) not that impressive.
While horsepower is displayed on a dyno chart, it NOT directly measurable on a dyno. The horsepower curve on all dynos is generated based on a mathematical calculation from the torque. It's a made up number.
While horsepower is displayed on a dyno chart, it NOT directly measurable on a dyno. The horsepower curve on all dynos is generated based on a mathematical calculation from the torque. It's a made up number.
A non-direct measurement doesn't mean it isn't being measured, hence the dyno run is done way above the rpm of the peak torque.
Road Master
Joined: Dec 2005
Posts: 1,078
Likes: 5
From: Hickory, NC
The history of where the horsepower unit of measure came from is a pretty interesting read. It was developed as a marketing ploy by steam engine companies to compare the work of their engine to that of a single horse (so they could sell more steam engines).
One horsepower is based on some guy estimating how many times a horse could turn a mill wheel in an hour. And, since no two horses are the same, it seems pretty arbitrary and made up to me. But, that's just my opinion. I certainly don't expect everyone to agree with it.
One horsepower is based on some guy estimating how many times a horse could turn a mill wheel in an hour. And, since no two horses are the same, it seems pretty arbitrary and made up to me. But, that's just my opinion. I certainly don't expect everyone to agree with it.
Last edited by FBFletch; Oct 14, 2011 at 11:59 PM.
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